The biotransformation of many drugs, other foreign compounds, and endogenous lipid-soluble substances is catalyzed by an enzyme system located in the smooth endoplasmic reticulum of the liver, which responds adaptively to challenge by these agents. A key component of the hepatic drug-metabolizing system is cytochrome P450, in that its concentration in the hepatic parenchymal cell may determine the overall rate of biotransformation. Adaptive regulation of cytochrome P450 appears to involve enzyme induction, which may be modulated by a number of metabolic and humoral factors. In the proposed project a new in vitro system, monolayer culture of adult rat hepatic parenchymal cells, will be employed to investigate regulation of cytochrome P450 in rat liver. With this culture system, the steady-state kinetics of cytochrome P450 in liver cells will be studied, and attempts will be made to characterize the mechanisms by which a number of putative regulatory factors affect the level of this hemoprotein. These include heme, heme precursors, various hormones, and nutritional factors. Moreover, we will investigate the mechanism of cytochrome P450 degradation and particularly the manner in which its heme is converted to bile pigment. The proposed studies are expected to provide a basis for a better understanding of the control of drug metabolism in the liver. BIBLIOGRAPHIC REFERENCES: Bissel, D.M. Formation and elimination of bilirubin. Gastroenterology 69: 519-538, 1975. Bissel, D.M., Hammaker, L. E. Hepatic delta-aminolevulinic acid synthetase: its regulation by microsomal heme. Clinical Research 23: 384A, 1975.